
Hmm, if the boss mob was kept as a pet, it would go on eating humans as well. Unfortunately not as tamed as Mac became. Also it showed a cute side only cuz of the MC's abilities, otherwise she would be no different from an enemy either. Sure, it was the system's fault that they invaded a different world, but compromising lives is a whole different matter. Not to mention, if they didn't defeat it, probably wouldn't be Allowed to go outside due to the message issued to them by the System. So MC weighed between saving her life and somehow giving up other lives (in place of humans) to feed it Or Clear the dungeon by killing her. That's why he said those words. I did feel bad as well, but they needed to clear it out eventually as others could also do it with stronger forces showing no Mercy. In the end, MC is also human.
You could’ve kept it as a pet nooo (˚ ˃⌓˂)